Floyd Mayweather will step back up in weight to fight Miguel Cotto for his WBA light-middleweight title on 5 May at the MGM Grand in Las Vegas.
The fight ends speculation that Mayweather would fight Manny Pacquiao in a super-fight in May.
"It will be a challenge for me to compete with him at this weight, but this is the type of test I thrive on," said Mayweather, 34.
Cotto responded: "On 5 May I will convincingly beat Floyd Mayweather."
He added: "I am here to fight the biggest names in boxing. I've never ducked anyone or any challenge in front of me."
Mayweather is due to serve an 87-day prison sentence for a domestic violence conviction but the Las Vegas authorities have deferred his sentence until 1 June and granted him a one-fight licence to allow the Cotto fight to take place.
"Miguel Cotto is a world-class fighter who can never be taken for granted and continues to prove he is one of the best in boxing," added Mayweather.
"I have no doubt in my mind that my title belt collection will increase once again."
Cotto, 31, who has a 37-2 record, successfully defended his WBA super welterweight crown with a revenge stoppage win over Antonio Margarito in December.
Mayweather, meanwhile, scored a fourth round stoppage of Victor Ortiz last September.
The 34-year-old, who holds a 42-fight unbeaten record, last fought at light-middleweight when he outpointed Oscar De La Hoya in a disputed split decision in 2007.